LAUSR

Prokaryotic adaptive immunity by CRISPR‐Cas systems, which confer resistance to foreign genetic elements, has been used by bacteria to combat viruses. To cope, viruses evolved multiple anti‐CRISPR proteins, which can inhibit system function through various mechanisms. Although the structures and mechanisms of several anti‐CRISPR proteins have been elucidated, those of the AcrIF9 family have not yet been identified. To understand the molecular basis underlying AcrIF9 anti‐CRISPR function, we determined the 1.2 Å crystal structure of AcrIF9. Structural and biochemical studies showed that AcrIF9 exists in monomeric form in solution and can directly interact with DNA using a positively charged cleft. Based on analysis of the structure, we suggest part of the anti‐CRISPR molecular mechanism by AcrIF9.